MySql, InnoDB & Null Values

Formerly I was using MyISAM storage engine for MySql and I had defined the combination of three fields to be unique.

Now I have switched to InnoDB, which I assume caused this problem, and now NULL != NULL.

So for the following table:

ID (Auto) |  Field_A   | Field_B  | Field_C

I can insert (Field_A,Field_B,开发者_开发百科Field_C) Values(1,2,NULL) (1,2,NULL) (1,2,NULL) infinitely many times.

How can I prevent this behavior?

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Depends on the business rules, but the first idea would be to set field_a and field_b as the primary key for the table. An AUTO_INCREMENT column can be used for a non-primary key column, but the key attribute has to be associated with the auto_increment column:

CREATE TABLE your_table (
  ID int auto_increment not null,
  Field_A VARCHAR(45),
  Field_B VARCHAR(45),
  Field_C VARCHAR(45),
  key(ID), --without this, you'll get MySQL ERROR #1075
  primary key(field_a, field_b)
);

The other alternative is to add a unique constraint (MySQL implements them as an index):

CREATE UNIQUE INDEX blah_ind USING BTREE ON your_table(field_a, field_b)